Fine roots, structural roots and water uptake

A tree’s root system is not made up of a set of roots that all have the same function. The roots closest to the trunk give stability and conduct water and nutrients. From them arise progressively finer branches, which explore the soil and account for much of the uptake.

The distinction between fine and structural roots is useful for explaining how the tree works, but it should not be treated as an absolute division. A root changes function as it grows and thickens, and two species can have roots of similar diameter with different physiological characteristics.

This difference helps to understand what happens during transplanting. The cut does not affect anchorage alone. It also temporarily reduces the extent of the network that supplies the crown.

The roots of largest diameter are found next to the trunk. These distribute forces into the ground, provide part of the stability and act as routes for transport and storage.

As they move away from the trunk, they divide into lateral roots and successive orders of branching. At the ends are younger roots, of smaller diameter and more active in exploring the soil.

For a long time, research grouped as ‘fine roots’ all roots below a given diameter, often one or two millimetres. This classification is simple, but it can lump together structures with different functions.

In a study of nine North American tree species, Pregitzer and other researchers analysed more than 18,000 root segments. The results showed clear differences between branching orders and between species. The most distal roots, close to the ends, held most of the absorbing length and had different chemical characteristics from the roots of higher orders.

To understand function, then, what matters is not just the thickness but also the position of the root within the network.

The youngest roots explore the pores of the soil and increase the surface of contact between the tree and the ground.

It is through this network, in association with mycorrhizal fungi, that an important part of the uptake of water and nutrients takes place. The activity is not limited to the visible tip of the root, but it tends to be higher in young, lightly suberised tissues.

These roots are shorter-lived than structural roots. They can die and be replaced as moisture, temperature, nutrient availability and the occupation of the soil change. The tree maintains a dynamic network, adapting growth to the zones where it finds the best conditions.

When part of the ground dries out, the roots do not move. The tree can reduce activity in that zone and develop new branches elsewhere, provided there is accessible soil and conditions for growth.

The presence of fine roots inside the rootball is therefore an important indicator in the preparation of a nursery tree. Those are the roots that will be available immediately after planting.

The structural roots leave the base of the trunk and spread through the soil. As well as carrying water and dissolved substances, they bear part of the forces transmitted by the crown and by the wind.

Stability does not depend on a single vertical root. In most established trees, anchorage comes from a network of lateral roots, from their connection with the soil and from the way the system developed in response to the ground.

The pattern varies with the species and with the conditions. Shallow, compacted or saturated soils can limit growth at depth and favour a shallower distribution. Obstacles, trenches, paving and differences in texture also deflect the roots.

When a mature tree is lifted, some structural roots are inevitably cut. The rootball preserves the part closest to the trunk and keeps the specimen stable until planting. Staking or temporary anchoring helps during the period when the new roots do not yet occupy the surrounding soil.

The image of a crown mirrored below ground is misleading.

Roots grow where they find compatible oxygen, water, temperature and physical resistance. In many soils, a significant part of the system is concentrated in the upper layers, where there is more aeration and biological activity.

That does not mean that all roots are shallow, nor that the depth is always small. Some species and some soils allow more vertical exploration. The important point is that the form of the roots cannot be predicted from the form of the crown alone.

On site, this knowledge has practical consequences. A very deep, narrow planting hole does not necessarily offer good conditions. It is usually more useful to prepare a wide area, correct compaction and ensure continuity with the surrounding ground.

Water moves from the soil to the roots in response to differences in water potential. It then enters the conducting tissues and travels on to the trunk, the branches and the leaves.

The tree loses water mainly through transpiration. When the stomata of the leaves are open, a tension forms that helps to pull the column of water through the xylem. The system depends on continuity: water available in the soil, contact with functional roots and conducting vessels able to supply the crown.

As the soil dries, water becomes harder to extract. Contact between some roots and the soil particles decreases, and resistance to movement increases. The tree can close its stomata and reduce transpiration, but that protection also limits photosynthesis and growth.

When the soil is waterlogged, the problem is different. Water fills the pores where there should be air. The lack of oxygen reduces the functioning and growth of the roots, even with plenty of water present.

That is why similar symptoms can have opposite causes. A wilted leaf can indicate a lack of available water, but also root suffocation in a saturated soil.

In a compacted soil there are fewer large pores, less air circulation and greater physical resistance to root expansion.

Water may infiltrate slowly and pool at the surface. In other cases, it moves through cracks and does not evenly wet the volume where the rootball sits. The appearance of the surface does not, in itself, tell you whether the roots received enough water.

Fine roots struggle to colonise very dense zones. They may concentrate next to the planting hole or follow interfaces between materials, without occupying the volume needed to sustain the tree in the long term.

Ground preparation should create continuity, and not just a compartment of loose soil around the rootball.

When a tree is lifted, structural roots are cut, along with a large quantity of fine branches that developed outside the area of the rootball.

The tree is left with less capacity for uptake and with temporarily reduced anchorage. It is this imbalance that explains the establishment period after planting.

The transplanting carried out during production helps to concentrate new roots close to the trunk. In a trial with Picea pungens, trees subjected to root pruning showed a greater quantity of root within the harvested rootball several years later. The result is not the same in every species, but it demonstrates the technical reason for the operation.

Root quality does not remove stress. It gives the tree more functional root with which to begin adapting.

During establishment, watering should reach the whole rootball and the surrounding soil.

Frequent shallow watering may keep only the first few centimetres moist. A mature tree needs volumes large enough to wet the mass of soil where the roots are. After that, the interval should allow the ground to drain and regain oxygen.

The rhythm depends on the species, the climate and the drainage. It cannot be defined by the size of the crown alone or by a weekly calendar.

At Diaplant, we recommend confirming the moisture in the ground instead of assessing the surface alone. Small observation points of about 20 cm next to the rootball help to see whether the water is getting there and whether the soil stays saturated for too long.

After planting, the fine roots begin to explore the prepared soil. Over time, some thicken and come to play a greater part in transport and stability. New absorbing roots go on forming at the ends.

Recovery depends on being able to grow out of the rootball. If there is a very abrupt transition, compaction or a lack of water, expansion slows. When the ground offers adequate porosity, moisture and oxygen, the root network increases progressively.

The first year is especially sensitive. The second still calls for attention in the hottest periods. Self-sufficiency increases as the tree occupies a larger volume of soil.

This article develops the functional side of the root system. The guide on transplanting explains how Diaplant prepares the roots throughout production.

Why we transplant trees within the nursery What happens to the roots after transplanting

1. Pregitzer, K. S.; DeForest, J. L.; Burton, A. J.; Allen, M. F.; Ruess, R. W.; Hendrick, R. L. (2002). “Fine Root Architecture of Nine North American Trees”. Ecological Monographs, 72(2), 293–309. DOI: 10.1890/0012-9615(2002)072[0293:FRAONN]2.0.CO;2.
2. Watson, G. W.; Sydnor, T. D. (1987). “The Effect of Root Pruning on the Root System of Nursery Trees”. Journal of Arboriculture, 13(5), 126–130. DOI: 10.48044/jauf.1987.027.
3. Watson, G. W. (1987). “Soil Moisture and Absorption of Water by Tree Roots”. Journal of Arboriculture, 13(2).
4. European Arboricultural Standards (2022). European Tree Planting Standard.